Dilepton Measurements at STAR
arXiv:1305.5447v1 [nucl-ex] 23 May 2013
Frank Geurts (for the STAR Collaboration) Rice University, Houston, TX 77005, USA E-mail:
[email protected] Abstract. In the study of hot and dense nuclear matter, created in relativistic heavy-ion collisions, dilepton measurements play an essential role. Leptons, when compared to hadrons, have only little interaction with the strongly interacting system. Thus, dileptons provide ideal penetrating probes that allow the study of such a system throughout its space-time evolution. In the low mass range (Mll < 1.1 GeV/c2 ), the dominant source of dileptons originates from the decay of vector mesons which may see effects from chiral symmetry restoration. In the intermediate mass range (1.1 < Mll < 3.0 GeV/c2 ), the main contributions to the mass spectrum are expected to originate from the thermal radiation of a quark-gluon plasma as well as the decays of charm mesons. In the high mass range (Mll > 3.0 GeV/c2 ), dilepton measurements are expected to see contributions from primordial processes involving heavy quarks, and DrellYan production. With the introduction of the Time-of-Flight detector, the STAR detector has been able to perform large acceptance, high purity electron identification. In this contribution, we will present STAR’s recent dielectron measurements in the low and intermediate mass range for RHIC beam energies ranging between 19.6 and 200 GeV. Compared to electrons, muon measurements have the advantage of reduced bremsstrahlung radiation in the surrounding detector materials. With the upcoming detector upgrades, specifically the muon detector (MTD), STAR will be able to include such measurements in its (di-)lepton studies. We will discuss the future dilepton program at STAR and the physics cases for these upgrades.
1. Introduction Dileptons have long been proposed as one of the more crucial probes in the study of the hot and dense matter [1]. Leptons do not experience the strong force, and thus will have negligible final state interactions in a strongly interacting medium, with a mean free path that is much larger than the typical size of this medium. Electromagnetic probes such as the dielectrons are emitted throughout the evolution of a heavy-ion collision. A typical dielectron invariant mass spectrum, therefore, involves a plethora of sources, ranging from Dalitz decays, which dominate the low invariant mass range, to Drell-Yan pair production which dominates at masses above 3 GeV/c2 . In addition, dilepton contributions originate from vector mesons, open heavy-flavor decays, and thermal radiation emitted from a quark-gluon plasma (QGP). The subsequent stages in the evolution of a hot and dense nuclear system can be identified with certain ranges in the dilepton invariant mass spectrum. In the high invariant mass range (Mll > 3 GeV/c2 ), dileptons from the decay of the heavy quarkonia such as the J/ψ and Υ mesons provide a means to study deconfinement effects in the hot and dense medium. These contributions are on top of a continuum from primordial Drell-Yan pair production. In the intermediate mass range (1.1 < Mll < 3.0 GeV/c2 ), the production of dileptons is closely related to the thermal radiation of the QGP. However, at higher center-of-mass energies this signal
competes with significant contributions from open heavy-flavor decays such as c¯c → e+ e− X, where such charm contributions may get modified by the medium. The prominent sources of dileptons in the low mass range (Mll < 1.1 GeV/c2 ), in addition to the Dalitz decays, are the direct leptonic decays of the ρ(770), ω(782), and φ(1020) vector mesons. The in-medium modification of the spectral shape of these mesons may serve as signature of chiral symmetry restoration. The ρ meson is of special interest given that in thermal equilibrium its contribution to the low mass range is expected to dominate through its strong coupling to the ππ channel [2]. Moreover, its short lifetime of τ ∼ 1.3 fm/c, will make the spectral shape of this meson especially sensitive to in-medium modifications [3]. At SPS energies, the observed low-mass dilepton enhancement observed in both the CERES dielectron [3, 4] and NA60 dimuon data [5] could be explained in terms of in-medium effects on the spectral shape of the ρ meson. Moreover, the dimuon measurements by NA60 are found to favor significant broadening of the line shape over a mass-dropping model. At top RHIC energies, the PHENIX collaboration measured a significant enhancement in its dielectron measurements, with a strong pT and centrality dependence [6]. However, models that have been able to describe the measurements at SPS energies are unable to consistently describe the PHENIX results in the most central collisions. The intermediate invariant mass range, between the φ and J/ψ mesons, opens an important window to thermal radiation from the QGP. However, contributions from the earlier mentioned semileptonic decays of open heavy-flavor hadrons are significant and need to be accounted for. Direct photon measurements by the PHENIX collaboration in the range of 1 < pT < 4 GeV/c yielded a substantial elliptic flow, v2 , comparable to the v2 of hadrons [7]. Model calculations which include a dominant QGP thermal emission source significantly underpredicted the observed v2 . However, incorporating a more detailed evolution of the hadronic phase, which involves the hadronic flow fields at chemical and kinetic freeze-out, appeared to improve the description of the direct photon v2 reasonably well, while setting further constraints on the initial QGP temperatures [8]. Dilepton elliptic flow measurements as a function of pT have been proposed as an independent measure to study the medium properties [9]. The combination of certain invariant mass and transverse momentum ranges allows for different observational windows on specific stages of the expansion. Dileptons can be used to further probe the early stages after a collision and possibly constrain the QGP equation of state. The installation of the Time-of-Flight (TOF) detector [10] and the upgrade of its data acquisition (DAQ) system [11], allowed the STAR experiment to extend its large-acceptance particle identification capabilities and increase its DAQ rate. The TOF detector not only extends the reach of hadron identification to higher momenta, but also significantly improves the electron identification capability in the low momentum range. This, combined with the RHIC Beam Energy Scan (BES) program in 2010 and 2011, has put STAR in a unique position to measure dielectron spectra in the low and intermediate mass ranges from top RHIC beam energies down to SPS center-of-mass energies. In this paper, the preliminary dielectron results √ from the STAR experiment at top RHIC energy, sNN = 200 GeV, are discussed as well as the preliminary results at several BES energies. The low invariant mass measurements are compared with recent model calculations. This paper concludes with an outlook on the future of the STAR dilepton program. 2. Electron Identification and Background Reconstruction The electron identification for the results reported in the next sections involves the STAR Time Projection Chamber (TPC) and the TOF detector. The TPC detector is the central tracking device of the STAR experiment and it provides charged particle tracking and momentum measurement. The energy-loss measurements, dE/dx, in the TPC are used for particle identification. The TOF detector, with full azimuthal coverage at mid-rapidity, extends
S/B
Counts
the particle identification range to higher momenta. The combination of the TOF velocity information and the TPC energy loss allows for the removal of slower hadrons which contaminate the electron sample. The selection criteria for electron identification have been optimized for each RHIC beam energy. With an additional track momentum threshold of pT > 0.2 GeV/c, the electron purity in the minimum bias Au+Au analysis is 95%. The unlike-sign invariant mass distributions, which are reconstructed by combining electrons and positrons from the same event, contain both signal and background contributions. Especially in high-multiplicity events, the contribution of the combinatorial background is large, see Fig. 1, and two methods have been used to estimate the background. The mixed-event method combines electrons and positrons from different events with similar total particle multiplicity, vertex position along the beam line, and event-plane angle. While the statistical accuracy of the background description can be arbitrarily improved by involving more events, the mixed-event method fails to reconstruct correlated background sources. At the lower invariant masses, such correlated pairs arise from jets, double Dalitz decays, Dalitz decays followed by a conversion of the decay photon, or two-photon decays followed by the conversion of both photons [12]. A background estimation based on the like-sign method can account for such correlated contributions. Its drawback, however, is that the statistical accuracy is only comparable to the unlike-sign, i.e. the original raw mass spectrum. Moreover, the like-sign method will need to consider detector acceptance differences, in contrast to the (unlike-sign) mixed-event method.
Au + Au sNN = 200 GeV (MinBias)
106
unLikeSign 5
3
10
p + p s = 200 GeV Au + Au sNN = 200 GeV (MinBias)
102
Au + Au sNN = 200 GeV (Central)
unLikeSign (mix)
10
LikeSign
STAR Preliminary
10
LikeSign (mix)
4
10
1
103
10-1 10-2
STAR preliminary
102
-3
0
1
2
3
4 2
Mee (GeV/c )
Figure 1. (Color online) Unlike-sign, likesign, and mixed-event dielectron invariant mass distributions in Au+Au minimum √ bias collisions at sNN = 200 GeV.
10
0
0.5
1
1.5
2
2.5
3
3.5
Mee (GeV/c2)
Figure 2. (Color online) Signal-to-background ratios in p+p, and Au+Au central, minimum √ bias events at sNN = 200 GeV [13].
√ The like-sign method is applied throughout the full mass range in Au+Au at sNN = √ 19.6 GeV, in the low mass range for Mee < 750 MeV/c2 at sNN = 200 GeV, and for Mee < 900 MeV/c2 at 39 GeV and 62.4 GeV. Above these respective invariant mass thresholds, the mixed-event method is applied [13, 14]. The signal-to-background ratios for 200 GeV p+p and Au+Au are shown in Fig. 2. √ 3. Dielectron Measurements at sNN = 200 GeV The√ STAR experiment has recently published its results on dielectron measurements in p+p at s =200 GeV [15]. These results were based on 107 million p+p events taken in the 2009 RHIC run, with only a partially installed TOF system. The agreement between the measured
Au + Au
10 1 10-1
Data/Cocktail
dN / dMee (c 2/GeV)
yields and the expected yields from a range of hadronic decays, heavy-flavor decays, and DrellYan production, provided an important verification of the analysis methods. More recently, the STAR experiment significantly improved its p+p data sample by about 700 million events with a fully installed TOF, effectively doubling its dielectron acceptance.
sNN = 200 GeV
10-40% × 0.05
40-80% × 0.02
MinBias × 0.0002
Central × 1
STAR preliminary
10-2 10-3 10-4 -5
10
10-6
(1)
0-10% (Central)
3 2 1 0
(2)
10-40%
3 2 1 0
(3)
40-80%
3 (4) 2
10-7 10-8
(a)
line: hadron decay cocktail
0
0.5
-9
10
3 2 1 0
1
1.5
2
2.5
3
3.5
4
Mee (GeV/c2)
1 0 0
0-80% (MinBias)
0.5
1
1.5
2
2.5
3
3.5
4
Mee (GeV/c2)
Figure 3. (Color online) Dielectron invariant mass spectra for Au+Au collisions at √ sNN =200 GeV for different centrality selections (left panel) and the ratio of data to cocktail (right panel). The systematical uncertainties are indicated by boxes. √ The left panel of Fig. 3 presents for Au+Au collisions at sNN =200 GeV the centrality dependence of the invariant mass spectra in the STAR acceptance (|yee | < 1.0, |ηe | < 1, and pT > 0.2 GeV/c). The measured yields are compared to a cocktail simulation of expected yields where the hadronic decays include the leptonic decay channels of the ω, φ, and J/ψ vector mesons, as well as the Dalitz decays of the π 0 , η, η 0 mesons [13]. The input distributions to the simulations are based on Tsallis Blast-Wave function fits to the invariant yields of the measured mesons [12]. These functions serve as the input distributions for the Geant detector simulation using the full STAR detector geometry. The ρ meson contributions have not been included in the cocktails as it may be sensitive to in-medium modifications which are expected to affect this meson’s spectral line shape [16]. In the intermediate mass range, the c¯c cross section is based on Pythia simulations scaled by the number of nucleon-nucleon collisions [17]. The cocktail simulations are observed to overestimate the data in central collisions. This can indicate a modification of the charm contribution. However, the observed discrepancy is still consistent within the experimental uncertainties. In the right panels of Fig. 3, the ratios of the data to cocktail yields have been depicted for different centrality selections. A clear enhancement in the low mass range can be observed. As the charm contributions scale with the number of binary collisions, the total cocktail yield increases with centrality, and only little centrality dependence can be observed in the ratio plots. On the other hand, as can be seen in Fig. 4, a comparison of the dilepton yield dN/dMee in the range of 150 < Mee < 750 MeV/c2 scaled to the number of participants, Npart , appears to indicate an increase of the low-mass-range enhancement with increasing centrality. Such an
increase, albeit with large uncertainties, would agree with similar observations reported in [4]. Figure 5 shows the inclusive dielectron transverse mass slopes, in the intermediate mass √ range, measured in p+p and Au+Au at sNN =200 GeV. The mT slopes in this plot have been determined for 1.1 < Mee < 1.8, 1.8 < Mee < 2.8 GeV/c2 in Au+Au and 1.1 < Mee < 1.6, 1.6 < Mee < 2.9 GeV/c2 , respectively. These measurements are compared with the slope parameters of hadrons (circles) and charm/Drell-Yan subtracted dimuon measurements (open squares, [18]). While the p+p results (triangles) are consistent with Pythia calculations, the transverse mass slopes in Au+Au collisions (filled squares) are observed to be larger than those in p+p. This is indicative of a possible combination of both thermal dilepton production and charm modification. Future detector upgrades will allow STAR to further disentangle the potentially modified charm contributions and help provide improved measurements of the thermal QGP dilepton radiation.
dN/(N
part
dM) (GeV/c 2)-1
-6
80 ×10
STAR Preliminary
70
0.15
